This invention relates to a.c. rotary dynamoelectric machines of the squirrel-cage induction type and more particularly to such a machine which can be used as a rotary phase converter or as a motor.
In many localities, as in rural or remote parts of the country, three-phase a.c. utility power is either not provided or not available. Instead, only single-phase service may be supplied. This presents certain problems for utility customers. First, it may be necessary or preferred to operate three-phase equipment. For example, three-phase motors are both widely available, as in integral-horsepower sizes, and desirable because of their efficiency, operating characteristics, and also because three-phase power may at some future time become available. Thus, either three-phase service must be extended to the customer at considerable expense or some type of phase converter must be used to provide conversion of the single-phase power to three-phase power. Previously many rotary phase converters have required relatively expensive and unreliable electrolytic starting capacitors and associated starting relay or switching mechanisms. Moreover, the line voltage supply to these rotary phase converters frequently drops so low that to insure reliable starting an auxiliary or pony motor was required or a time-delay controller was needed together with a bank of starting capacitors, all of which greatly increases the expense of the converter.
Another type of problem which may be encountered, where it is required to operate from a single-phase power source, is that the peak current demand may be limited to a relatively low value. For example, a utility company typically may not permit a demand of greater than 260 amperes from a 230 volt single-phase line, or else may charge a considerable amount for higher demand service. Thus a utility customer desiring to operate a motor is severely restricted as to the size of motor which can be used, since normally a single-phase motor has a starting current which is several times (e.g., 5-6 times) its normal running current. In this example, the customer would be precluded from using a single-phase motor of greater than about 10 horsepower (such a motor having a starting current of 200-240 amperes). Still another difficulty is the need for these motors to start under very high torque loads, such as encountered in the operation of oil field pumps, punch presses and the like.
In order to improve the starting and operating characteristics of such rotary converters and motors, high resistance induction rotors were developed as disclosed in coassigned U.S. Pat. No. 3,670,238. There the squirrel-cage rotors have rotor bars and end rings with substantially reduced cross-sections so that the rotor resistance is two to three times greater than that of rotors of conventional commercial design. While converters and motors with these increased-resistance rotors have greatly improved characteristics, any substantial increase of rotor resistance by further reducing the cross-section of the bars or end rings is impractical because of the limiting effect on the current carrying capacity of these bars and end rings and the rotors loss of structural strength and integrity. Increasing the resistance of the induction rotor by forming the bars and end rings of higher resistance alloys would greatly increase material and fabrication costs and also be impractical in attaining the greatly increased rotor resistance parameters desired.